As the window glass sheet for architecture or automobile, a tempered glass sheet is extensively used as a safety glass sheet. As a method for manufacturing a tempered glass sheet, there is extensively practiced an air-cooling method of quickly cooling the glass sheet that has been heated to a temperature near the softening point to form a compressive stress layer in the surface of the glass sheet.
In view of the design and aerodynamic characteristics of automobiles, there are great demands for curved window glass sheets. Accordingly, in the air-cooling method the heated glass sheet is often formed into a curved shape before the glass sheet is cooled to temper. The method for bending a glass sheet includes a method of pressing a glass sheet suspended by a tong (a glass-suspender) with a pair of convex and concave press molds, and a method of pressing a glass sheet that has been transferred horizontally from a heating furnace with a pair of upper and lower press molds.
In the bending methods involving pressing with molds, the step of heating a glass sheet and the step of bending the glass sheet are basically taken as independent steps. However, there is proposed a method to practice the two steps as a single step. In such a method the glass sheet transferring means such as rolls or beds are provided in the heating furnace with a predetermined curvature, so that the heated glass sheet is gradually bent down by its own weight until it constitutes a curved surface having the curvature. The bent glass transferred to the cooling apparatus adjacent to the heating furnace is cooled into a tempered glass sheet. This method is excellent in the point that there is no necessity to press the glass sheet individually. In order to make use of this feature, various improvements on the method have been made for practical applications (e.g., Japanese Examined Patent Publications Nos. S44-14832/1969, S48-5242/1973, and Japanese Unexamined Patent Publication No. H7-237928/1995).
However, the tempered glass sheet by the method tends to be formed into a shape different from what originally was expected. Typically, as shown in FIG. 5A, when a flat tempered glass sheet is expected, the rear end part of the glass sheet in the direction of transfer becomes convex, and as shown in FIG. 5B, when a curved glass sheet that should show a convex upward shape was expected, the rear end part shows a larger curvature than the front end part in the direction of transfer.
Such a difference of curvature is caused mainly by the fact that the cooling air blown to the glass sheet in the cooling apparatus goes into the heating furnace. In other words, as shown in FIG. 6, when the glass sheet is transferred to the cooling apparatus, the cooling air blown to the upper surface of the front end part in the direction of transfer of the glass sheet runs along the surface of the glass sheet to the upstream side of transfer to go into the heating furnace. The cooling air lowers the temperature on the upper surface at the rear end part of the glass sheet before it reaches the cooling apparatus. On the other hand, the lowering of the temperature on the upper surface at the front end part of the glass sheet is not so remarkable as at the rear end part of the glass sheet, because the intrusion of the cooling air into the heating furnace becomes remarkable by the flow along the surface of the glass sheet, as shown in FIG. 6.
As a result, at the rear end part of the glass sheet in the direction of transfer, the temperature variation range in cooling becomes greatly different between the upper and the lower surfaces of the glass sheet and the shrinkage amounts by cooling also show differences. Thus, there has been a phenomenon that the curvatures of the glass sheet become different between the front end part and the rear end part in the direction of transfer.
In view of the above problem, in Japanese Utility Model Publication No. H4-41152/1992, there is proposed to dispose a heating device between the heating furnace and the cooling apparatus. According to the heating device, the inconvenience as referred to above can be alleviated by compensating the temperature difference on the surface of the glass sheet prior to it being transferred into the cooling apparatus. However, the method using the heating device necessitates heating a specified part of the glass sheet in the course of transferring it, so that the operation is not necessarily easy, and involves a problem to be overcome.